THE BEHAVIOR OF BACTERIA 35 



tilled water in the same way (Fig. 29). This result may be due to the 

 fact that the osmotic pressure of the distilled water is less than that of 

 the sea water. On the other hand, it is possible that it is due merely to 

 the cessation of the chemical action of certain components of the sea 

 water. The case would then be comparable to the reaction induced 

 when bacteria come to a region containing no oxygen, as described in 

 the preceding pages. 



Most bacteria do not react to light. But there are certain bacteria 

 for whose successful development light is required, and in these species 

 we find that reaction to light occurs in the same manner as the reaction 

 to oxygen in others. The species which react to light belong chiefly to 

 the group of sulphur bacteria. They contain a purple coloring matter 

 (bacterio-piirpurin), which acts in a manner analogous to the chlorophyl 

 of higher plants. By its aid, through the agency of light, these bacteria 

 break up and assimilate carbon dioxide, giving off oxygen. 



Engelmann (1882 a, 1888) made a thorough study of the relations to 

 light in one of these bacteria, Chromatium photometricum (Fig. 23, b). 

 This organism moves actively and develops well in diffuse light, but in 

 the dark movement soon ceases and development stops. Only in the 

 light does it assimilate carbon dioxide and give off oxygen. In corre- 

 spondence with this, Chromatium photometricum collects in lighted areas. 

 This takes place in the same manner as the collection of bacteria in oxy- 

 gen. Engelmann placed the bacteria on a glass slide, in the usual way, 

 then illuminated a certain spot from below, while light was cut off from 

 the remainder of the preparation. He found that the bacteria do not 

 react on entering the lighted area. But when once within this area, on 

 coming to the outer boundary they suddenly reverse their movement 

 and swim backward a distance. Then they start forward again; on 

 coming anew to the boundary they react as before, and this happens 

 every time they reach the confines of the lighted area. Thus none leave 

 the light ; all those that enter the lighted area remain, and a dense col- 

 lection is soon formed here. In every detail the phenomena are parallel 

 to those found in the reactions of other bacteria to oxygen, as described in 

 previous pages. 



A sudden decrease of light causes the same backward movement that 

 is observed when the bacteria come to the edge of the lighted area. If 

 the light is suddenly decreased by closing the diaphragm of the micro- 

 scope, all the bacteria at once swim backward a distance, — often ten 

 to twenty times their length. This shows that the reaction is not due 

 to the difference in illumination of two ends or two sides of the organism, 

 but only to the sudden decrease in light. This is shown also by the fact 

 that the bacteria may swim completely across the boundary of the lighted 



